The present invention relates to a medical appliance and more particularly to a medical appliance and method for bridging and stabilizing spaced apart bone segments.
While the present invention is intended for use with vertebrae, it should be understood that any spaced apart bone segment may utilize the present invention. The spine is a flexible structure comprised of thirty-three vertebrae. The vertebrae are separated and cushioned from each other by fibrous cartilage in structures called intervertebral disks. If the spine is injured or becomes diseased, surgical intervention involving removal of one or more of these disks and fusion of the adjacent vertebrae, may be indicated. Such disk injuries can happen in the neck, in the thoracic region and in the lumbar region. The more frequent injuries are in the lower lumbar and in the lower cervical regions.
Treatment of a herniated disk in the neck and in the lumbar region continues to be a challenging field of medicine. The classical treatment for a ruptured disk continues to be removal of the disk from between the vertebrae. By this process of removing the disk, overall spinal instability is increased. This may aggravate the patient to some degree after the operation. Another procedure previously employed is to replace the disk space with a bone graft, bringing about fusion of the vertebrae above and below the disk, eliminating the empty space between the vertebrae and improving stability.
Theoretically a diskectomy with fusion is a satisfactory procedure, though not ideal because the replaced bone does not have the principal functions of the cartilage tissue of the disk. This fusion procedure is technically demanding and has medical complications because of several physiological factors.
It must be remembered that the disk primarily serves as a mechanical cushion while permitting limited mobility. For any replacement system for a disk to be truly effective, it must allow for mobility within the natural limits of the original disk. In other words, the replacement should match appropriate joint rheology (movement behavior). The natural disk allows about 11 degrees of flexion-extension, limited lateral bending of 3 to 5 degrees, and very restricted rotation of about 1 degree.
Various prosthetic devices and implants are disclosed in the art, but all are characterized by compromises to the full functions of a natural disk discussed above. Examples of the prior art include the following U.S. Pat. Nos.: 5,893,890; 5,693,100; 5,658,336; 5,653,761; 5,653,762; 5,390,683; 5,171,278; and 5,123,926. The specification and drawings of U.S. patent application Ser. No. 09/627,261 is herein incorporated b y reference. The present invention provides stabilization of spaced apart bone segments while still allowing some flexion and rearward extension of the bones with some lateral displacement. The present invention is particularly useful to stabilize adjacent vertebrae in the human spine.
Accordingly, the present invention provides a medical appliance and method for bridging and stabilizing spaced apart bone segments. The present invention uses a rigid, elongate bottom plate to span space between bone segments and to attach to a ledge surface of each bone such that a plurality of screw barrels rotatably engaging a plurality of openings in the bottom plate base may receive a plurality of barrel screws to couple the bottom plate to a rigid, elongate top plate. The top plate is attachable to an outside surface of each bone segment and has a top plate base having a plurality of slots for rotatably receiving screw barrels. The slots are aligned to allow barrel screws to engage the screw barrels such that the top and bottom plate may be coupled together.
The top plate is also equipped with a plurality of countersinks. The countersinks receive bone screws such that the top plate may be attached to the outside surface of each bone segment. The present invention also has a locking member disposed in the countersink or attached to the bone screw for securing the bone screws to the medical appliance such that axial and rotational movement of the bone screws is restricted.
The screw barrels of the present invention may be equipped with an angled first end and a riveted second end such that movement of the screw barrels may be restricted in a first and second direction while still allowing the screw barrels to rotate within the openings to allow the barrel screws to engage the screw barrels at various angles. A locking ring having a plurality of tapered rachet teeth on an interior surface may be used to secure the bone screws to the medical appliance. The teeth engage a serrated neck of each bone screw upon insertion of each bone screw into a countersink, thus securing the bone screws.
The configuration of each locking ring and each countersink may vary depending on the application of the medical appliance. They may have a substantially circular configuration or a non-circular configuration that allow them to work in concert to provide varying degrees of axial and rotational movement for the bone screws. In one embodiment of the present invention, the locking ring may be attached to the bone screw to provide a preassembled locking member.
Another embodiment of the present invention provides a pre-assembled locking member having an arcuate lower surface such that the bone screw is capable of flexion. Still another embodiment of the present invention provides for a locking member comprising a bone screw having a grooved collar engageable with a threaded inner surface of the countersink.